Apple has accelerated the Mac OS X 10.5.5 seeding with build 9F17 delivered to developers last night. The latest build still lists no known issues and provides fixes for iCal, Mail Sync, Back to My Mac and Core Animation.
While difficult to predict when Apple will release this version to the public, accelerated releases with "no known issues" tend to be signs of an impending release.
Apple last updated Mac OS X with version 10.5.4 on June 30th, 2008. The next release will be a free update and is expected to incorporate over a hundred bug fixes.